The majority of studies, including our own preliminary results, support the model that the cells that repair the kidney after acute injury are existing tubule epithelial cells. As proposed in the background and preliminary results, we believe that de-differentiation of these cells is central to their ability to survive and mediate tubule repair. However, an alternative view of epithelial de-differentiation has emerged that suggests that this process may instead lead to true epithelial-mesenchymal transformation (EMT), thereby promoting fibrosis and preventing repair. It is our hypothesis that kidney injury triggers multiple stimuli that lead to the up-regulation of the helix-loop-helix (HLH) transcription factors Twist, Snail and Slug in the injured epithelial cell, with subsequent expression of the downstream factor Foxc2 which in turn activates the genetic program of epithelial de-differentiation. We postulate that under the proper conditions this de-differentiation is followed by increased expression of the transcriptional regulator p204 and downregulation of the HLH transcription factors, thus promoting re-differentiation. According to this hypothesis, sustained epithelial de-differentiation or true EMT would result if failure of HLH downregulation or p204 upregulation were to occur, while interventions to decrease expression of the appropriate HLH factor or increase p204 expression would be predicted to prevent and/or reverse EMT. To test this hypothesis, over-expression and/or knock-down of the candidate HLH factors and of p204 will be performed in cultures of proximal tubule cells, and the effects on de-differentiation, redifferentiation, cell survival, and morphogenic responses determined (SAs 1A-C and 2A-B). Factors that are found to directly regulate epithelial de-differentiation and re-differentiation in these in vitro experiments will then be tested in vivo using an antisense oligonucleotide approach to determine their role in mediating epithelial cell repair and/or renal fibrotic responses (SAs 1D and 2D). Finally, we propose to use genetic labeling of tubule cells to track the fate of those cells that undergo de-differentiation and quantitatively determine their contribution to tubule repair. In addition, this approach will allow us to isolate these cells at specific points in the de-differentiation and re-differentiation process for gene expression profiling to identify novel pathways that are likely to regulate the outcomes of epithelial survival and re-differentiation, or sustained de-differentiation and progressive fibrosis (SA 3). PUBLIC HEALTH RELEVANCE The studies in this proposal are designed to help us understand how the kidney repairs itself after injury. We will use cultured cells and mouse models of kidney injury to find out how the cells that survive the injury are able to grow, divide and repair the injury so that the kidney can function normally again. We believe that this information will help us to develop treatments to improve kidney repair in patients who are very ill, and thus decrease the need for dialysis and improve patient survival.